Central heating thermostat

[Alan Stirrup]

Hi all, my condenser boiler central heating has started to do strange things so I set about trying to diagnose the problem. When I got to the thermostat I found that there was no power to it, could anybody tell me where this is powered from please? It is an old system, almost 30 years old so in reality it could do with being replaced but I can hardly afford to switch it on let alone replace it!

 

[freddo]

The power to the thermostat is likely only to be present if the time clock for the heating is set to on.

 

[davesparks]

When I got to the thermostat I found that there was no power to it, could anybody tell me where this is powered from please?

You need to give us some details for us to be able to help with this.
What controls do you have for the system? Is it a combi boiler or a conventional system with motorised valves etc?
Do you have a seperate programmer/timeswitch and thermostats or something else?

Also what are you using to test for voltage? And what points did you test between?

In a conventional heating control circuit the room stat is wired in series with the CH channel of the programmer. So the room stat receives power only when the programmer CH channel is on.

 

[Alan Stirrup]

The power to the thermostat is likely only to be present if the time clock for the heating is set to on.

Thanks for the reply Freddo, I tried it with the CH both on and off, this was the reason I posted as I can't figure out what else it could be?

 

[buzzlightyear]

Thanks for the reply Freddo, I tried it with the CH both on and off, this was the reason I posted as I can't figure out what else it could be

post some pics up of the set up .

 

[buzzlightyear]

I tried it with the CH both on and off

either the time clock or the wall thermostat .

 

[Alan Stirrup]

You need to give us some details for us to be able to help with this.
What controls do you have for the system? Is it a combi boiler or a conventional system with motorised valves etc?
Do you have a seperate programmer/timeswitch and thermostats or something else?

Also what are you using to test for voltage? And what points did you test between?

In a conventional heating control circuit the room stat is wired in series with the CH channel of the programmer. So the room stat receives power only when the programmer CH channel is on.

Hi Dave, separate timer/thermo, conventional Baxi boiler. I’ve tried to attach pics of the thermostat but each pic was too large even with adjustments. The thermostat appear to be called T.R.A.C if that makes any sense?

 

[Alan Stirrup]

Hi Dave, separate timer/thermo, conventional Baxi boiler. I’ve tried to attach pics of the thermostat but each pic was too large even with adjustments. The thermostat appear to be called T.R.A.C if that makes any sense?

Also, if it helps, the pump seems to run continuously even when the CH has been off for hours.

 

[brianmoooore]

Faulty pump over run control?

 

[freddo]

Zone valve stuck open? open

 

[Lucien Nunes]

We really need pics of the wiring both at the thermostat and preferably at the wiring centre. Perhaps you could switch the resolution of the camera down to a lower setting and retake them?

 

[Alan Stirrup]

Hi all, I managed to resize the images, let me know if you need any more.

 

[timhoward]

I’m suspicious that the connection on the right in the photo above is not a good connection.

But in general you need to follow it through one stage at a time.
Time clock. Wall stat. Zone valve (3 port) or zone valves (2 x 2 port). Pump. Boiler

There should be a junction box, usually in the airing cupboard where all of this comes together and 5-10 minutes with a 2 pole tester is normally enough to work out what is going wrong.
A photo of this box is rather important to being able to help more.

 

[Lucien Nunes]

I’m suspicious that the connection on the right in the photo above is not a good connection

Yes it looks like the screw is clamped onto the insulation. In practice there is usually continuity but it should be corrected.

The OP mentioned the boiler doing 'strange things'.... what exactly is the fault that led to investigating the room stat?

 

[Alan Stirrup]

Thanks guys, I'll try and upload a pic of the junction box later today. That connection has been like that since the heating was installed nearly 30 years ago, the issue now is there is no power in the cables. The issue that prompted me to check the thermo was the temperature upstairs was intolerable and the pump was running continuously even when the heating had been off for several hours.

 

[Lucien Nunes]

That connection has been like that since the heating was installed nearly 30 years ago, the issue now is there is no power in the cables.

Badly made connections can eventually go open circuit, as the surfaces where the wire is just barely touching oxidise over time. If you were checking for presence of a feed by touching your meter probe on the screw, loss of continuity would cause a zero reading, as that is probably the incoming feed from the programmer. (I expect the stat output is the yellow core oversleeved in red)

We seem to have three different symptoms with no certainty of how they relate to each other.

• Room continues heating when it shouldn't
• Pump runs when it shouldn't
• No feed to room stat

Definitely need to see inside the wiring centre and, if it's not obvious from that, the configuration of motorised valves (Y-plan, S-plan etc.)

 

[timhoward]

The issue that prompted me to check the thermo was the temperature upstairs was intolerable and the pump was running continuously even when the heating had been off for several hours.

I have a feeling this could be two faults.
Pure speculation but my guess would be that (as already mentioned above) initially the zone valve head stuck open. That could lead to boiler and pump being on 24/7 with only the overheat stat on the boiler to possibly interrupt proceedings.
In turn being on perpetually could have then helped an old time clock to give up the ghost.
Complete guesswork though.

The first test is whether when you ask the time clock to call for heat, a terminal in the wiring centre becomes live.
If nothing changes in the wiring centre then the time clock itself is the first suspect.

Also, do you have one or two zone valves?
With everything off, does the little lever move freely on one of them (or the only one)?

 

[Alan Stirrup]

I have a feeling this could be two faults.
Pure speculation but my guess would be that (as already mentioned above) initially the zone valve head stuck open. That could lead to boiler and pump being on 24/7 with only the overheat stat on the boiler to possibly interrupt proceedings.
In turn being on perpetually could have then helped an old time clock to give up the ghost.
Complete guesswork though.

The first test is whether when you ask the time clock to call for heat, a terminal in the wiring centre becomes live.
If nothing changes in the wiring centre then the time clock itself is the first suspect.

Also, do you have one or two zone valves?
With everything off, does the little lever move freely on one of them (or the only one)?

I'll run a test on the time clock, Tim, hopefully this will give us a better indication of what's going on. I have one zone valve and the lever moves freely without resistance.

 

[timhoward]

I have one zone valve and the lever moves freely without resistance.

If this is the case without calling for hot water or central heating AND it’s the Honeywell style unit I’m imagining then I think you have a problem there as the lever should be quite tough to move with everything off. It should feel as though you are overcoming a strong spring. It sounds as though it’s stuck.

With power off, the cover comes off with a small screw near the lever. If you take the cover off I’d imagine you could then see if it’s sticking or not. The gear teeth may look shiny and worn.

If required, the head is a fairly easy replacement as long as you note where the wires go, part is available at Screwfix. Sadly you need the more expensive 3 port head.

(I appreciate you are not looking for extra expense but if it were my house I’d get a plumber to change it to 2 x 2 port valves. The heads are much cheaper for future replacement and 3 port valves can leave the motor running perpetually for hours on end after one permutation of actions. )

 

[Lucien Nunes]

If this is the case without calling for hot water or central heating AND it’s the Honeywell style unit I’m imagining then I think you have a problem there as the lever should be quite tough to move with everything off.

Assuimng this is a 3-port valve, (did we confirm that explictly?) this is true only if the last demand was for water, which would leave the valve idle in the spring refurn (B) position if the last demand was for heating the valve will be parked in the A position with the motor energised and the lever will be free to move. Briefly demanding hot water only, should release it back to the B position.